The present disclosure relates to wireless digital communication systems, and more particularly to estimation of channel characteristics and interference level in such systems.
Demand for wireless digital communication and data processing systems is on the rise. Inherent in most digital communication channels are errors introduced when transferring frames, packets or cells containing data. Such errors are often caused by electrical interference or thermal noise. Data transmission error rates depend, in part, on the medium which carries the data. Typical bit error rates for copper based data transmission systems are in the order of 10-6. Optical fibers have typical bit error rates of 10-9 or less. Wireless transmission systems, on the other hand, may have error rates of 10-3 or higher. The relatively high bit error rates of wireless transmission systems pose certain difficulties in encoding and decoding of data transmitted via such systems. Partly because of its mathematical tractability and partly because of its application to a broad class of physical communication channels, the additive white Gaussian noise (AWGN) model is often used to characterize the noise in most communication channels.
Data is often encoded at the transmitter, in a controlled manner, to include redundancy. The redundancy is subsequently used by the receiver to overcome the noise and interference introduced in the data while being transmitted through the channel. For example, the transmitter might encode k bits with n bits where n is greater than k, according to some coding scheme. The amount of redundancy introduced by the encoding of the data is determined by the ratio n/k, the inverse of which is referred to as the code rate. Codewords representing the n-bit sequences are generated by an encoder and delivered to a modulator that interfaces with the communication channel. The modulator maps each received sequence into a symbol. In M-ary signaling, the modulator maps each n-bit sequence into one of M=2n symbols. Data in other than binary form may be encoded, but typically data is representable by a binary digit sequence.
Often it is desired to estimate the channel and the level of interference. On the forward link (FL), common pilot symbols are known to have been used. In orthogonal frequency division multiplexing (OFDMA) systems, such common pilot symbols are typically scattered over the entire bandwidth shared by all the users. In a traditional single-antenna transmission, such common pilot symbols may be exploited by all the users for the purpose of FL channel estimation. The bandwidth and channel coherence time values that are typical in cellular applications render common pilot tones particularly useful. However, common pilot symbols are broadcast to all the users and, therefore, are not adapted to carry user-specific signature.